FIG. 1 is a block diagram of a conventional pulsed EMI metal detector and method of operation. A current loop transmitter 10 is placed in the vicinity of the buried metal target 12, and a steady current flows in the transmitter 10 for a sufficiently long time to allow turn-on transients in the soil (soil eddy currents) to dissipate. The transmitter loop current is then turned off. The transmitter current is typically a pulsed waveform. For example, a square-wave, triangle or saw-tooth pulsed waveform, or a combination of different positive and negative current ramps.
According to Faraday's Law, the collapsing magnetic field induces an electromotive force (EMF) in nearby conductors, such as the metal target 12. This EMF induces eddy currents to flow in the conductor. Because there is no energy to sustain the eddy currents, they begin to decrease with a characteristic decay time that depends on the size, shape, and electrical and magnetic properties of the conductor. The decay currents generate a secondary magnetic field that is detected by a magnetic field receiver 14 located above the ground and coupled to the transmitter 10 via a data acquisition and control system 16.
Pulse induction metal detector (PIMD) antennas (transmitter and receiver coil) come in two basic types as shown in FIGS. 2a and 2b. The first type of PIMD shown in FIG. 2a illustrates a single combined transmitter and receiver coil 23 and damping resistor 22 with multiple loops of wire forming the coil 23. A current pulse is sent through the multiple turn coil 23 and the received metal detection signal is sensed by the same coil 23. The small voltage generated by the metal target is typically amplified by a high gain electronic amplifier 25 (typical gain factor of 100 to 1000). A protection circuit 24 is provided to protect the sensitive amplifier from the high kick-back voltage pulse generated by switching the inductive coil off abruptly (V=L di/dt, where L is the inductance of the transmitter coil and di/dt is the slope of the current decay in the coil).
The second type of PIMD illustrated in FIG. 2b uses a separate coil 27 and damping resistor 26 for the transmitter and a coil 29 and damping resistor 28 for the receiver. This configuration provides isolation between the transmitter circuit and the receiver circuit and allows for more flexibility in the receiver coil 29 (e.g., different number of turns, size or differential coil configuration) and amplifier circuit design (e.g., single ended operation of electronics). The high gain amplifier 25 also sees the high kick-back voltage pulse generated by switching the transmitter coil 27 off abruptly and protection circuitry 24 is needed to protect it from damage.
The induced eddy currents in a metal target are proportional to the change in magnetic field with time (ΔB/Δt) at the metal target location. For high sensitivity, one would like to have dB as large as practical and Δt (the change in time) matched to the metal object's time response (bandwidth). For a small metal object with a fast time response (high bandwidth) the optimal detector sensitivity would be achieved with a small Δt matched to the small metal objects response (matching bandwidth of sensor and target). For a large metal object with a slower time response the optimal detector sensitivity would be achieved with a larger Δt matched to the metal object's time response. The magnetic field (B) is proportional to the current (I) in the transmitter coil and the number of coil turns (N), thus B˜IN. More coil turns (N) increases the magnetic field at the target depth for a fixed current. However, increasing the number of coil turns also increases the kick-back voltage across the transmitter coil and switch due to the increased inductance. The voltage across the transmitter coil and the electronic switch turning off the coil current is V=L di/dt and L˜N2. More coil turns also increases the capacitance C of the coil due to the potential (voltage) differences that exist between the individual turns of wire which makes up the coil.
Consider the transmitter coil. The same effects apply also to a receiver coil that is being excited by a transmitter coil. At the moment of current change in the transmitter coil, a high voltage appears across the coil. A fixed shunt resistor R is typically placed across the transmitter coil to dissipate the current in the coil. The resistor is called the damping resistor since it is used to dampen or suppress coil oscillation caused by the LCR circuit formed by the coil. The larger the shunt resistor, the greater the current dissipation and the faster the current decay. Fast current decay allows for small metal targets to be more easily detected since the coil has a higher bandwidth. If the damping resistor value is set to high current is forced into the coil where the capacitance and inductance combination causes voltage/current oscillations: the oscillations will mask small metal target signals. A small damping resistor slows down the coil decay and lower the sensitivity of the coil to small metal targets. Controlling the damping resistor effects the performance of the PIMD.